1. Technical Field
The present invention relates to semiconductor light-emitting devices and to methods of manufacturing semiconductor light-emitting devices.
2. Description of the Related Art
In Japanese Unexamined Pat. App. No. H06-268257, a gallium-nitride-based compound semiconductor light-emitting device employing a sapphire substrate is described. An object of the invention set forth in Pat. App. Pub. No. H06-268257 is to afford a nitride semiconductor light-emitting device in which a multilayer film stack composed of well and barrier layers of equal thickness is rendered a single emission layer to mitigate lattice mismatch and improve the crystallinity of the emission layer. In Embodiment 1 of the publication, the barrier and well layers are grown at 800 degrees centigrade. The well layers are composed of 2-nm thick In0.2Ga0.8N, while the barrier layers are composed of 2-nm thick In0.04Ga0.96N. In Embodiment 2 of the publication, the barrier and well layers are grown at 800 degrees centigrade. The well layers are composed of 5-nm thick In0.2Ga0.8N, while the barrier layers are composed of 5-nm thick In0.04Ga0.96N.
Japanese Unexamined Pat. App. Pub. No. H08-316528 describes a nitride semiconductor light-emitting device employing a sapphire substrate. The invention set forth in Pat. App. Pub. No. H08-316528 is directed to making available not only green LEDs, but also nitride semiconductor light-emitting devices that exhibit high brightness and high output power at emission wavelengths of 360 nm or more. A nitride semiconductor light-emitting device of Embodiment 10 therein has a quantum-well structure consisting of 1-nm thick undoped In0.15Ga0.85N well layers, and of 1-nm thick undoped In0.05Ga0.95N barrier layers. A nitride semiconductor light-emitting device of Embodiment 11 of the publication has a quantum-well structure consisting of 2.5-nm thick undoped In0.15Ga0.85N well layers, and of 5-nm thick undoped In0.05Ga0.95N barrier layers. These nitride semiconductor light-emitting devices have emission wavelength of 415 nm.
Increasing the indium composition in the active layer harms the crystallinity, which is prohibitive of obtaining inter-band emission of green light at a wavelength in the vicinity of 520 nm. In the foregoing nitride semiconductor light-emitting devices, however, band-gap energy of the active layer is smaller than its original band-gap energy because tensile stress is applied to the active layer, with the high-thermal-expansion-coefficient active layer intervening between low-thermal-expansion-coefficient cladding layers.
For this reason, the well layers in the light-emitting devices cannot be made thick.
Japanese Unexamined Pat. App. Pub. No. 2000-133883 describes a nitride semiconductor light-emitting device employing a sapphire substrate. An object of the invention set forth in Pat. App. Pub. No. 2000-133883 is to make available a nitride semiconductor light-emitting device enabling enhancement of photoelectric conversion efficiency without diminishing optical output power. In the embodiments therein, barrier and well layers are grown at 800 degrees centigrade. The well layers are composed of 3-nm thick In0.3Ga0.7N, and the barrier layers are composed of 30-nm thick Si-doped GaN.
In Japanese Journal of Applied Physics, Vol. 41, 2002, pp. L1431-L1433, a blue LED structure is described. For this blue LED structure, a low-temperature GaN buffer layer, undoped GaN layer, n-type GaN layer, MQW active layers, p-type AlGaN layer, and p-type GaN layer are formed by MOCVD successively onto a patterned c-plane sapphire substrate. The MQW structure is composed of 3-nm thick undoped In0.3Ga0.7N well layers, and of 25-nm thick undoped GaN barrier layers. The p-electrode is composed of Rh/Ir/Pt in mesh form. The blue LED structure is encapsulated with epoxy resin.
When a current of 20 mA (current density being 105 A/cm2 directly under the mesh electrode, and 39 A/cm2 in the active layer) is applied through a mesh electrode having a 70% aperture ratio, an output power of 18.8 mW is generated, and an external quantum efficiency of 34.9% is achieved. On the other hand, when a current of 10 mA (current density in the active layer being 195 A/cm2) is applied through the mesh electrode, an output power of 65 mW is generated, but nevertheless the external quantum efficiency drops to 25%.
Physica Status Solidi (a), Vol. 188, No. 1, 2001, pp. 15-21 describes a light-emitting diode of quantum well structure. The light-emitting diode is composed of a nucleation layer formed on the c-plane of a sapphire substrate at low temperature, an underlayer doped with Si, a MQW emission layer composed of InGaN/GaN, a p-type AlGaN layer, and a p-type GaN layer. A chip (area: 0.0007 cm2) that exhibits wavelengths of 428 nm, 454 nm, 501 nm, and 545 nm is flip-flop mounted, and 1-kHz pulse current with duty ratio of 1% is applied to the chip. When the current density is 12.5 A/cm2 (0.0875 A), power efficiency rises to 28% at the most efficient wavelength of 428 nm. On the other hand, when the current density is 125 A/cm2, the power efficiency drops to a level of 18%.
In Applied Physics Letters, Vol. 78, 2001, p. 2617, an InGaN/GaN-based light-emitting diode is described. According to this reference, in the InGaN-based light-emitting diode, an increase in well-layer thickness diminishes LED output power.
In the above-cited references, when the density of current applied to the light-emitting devices is low, their external quantum efficiency is high. The external quantum efficiency, however, decreases with increasing current density. In blue light-emitting diodes (of from 420 nm to 490 nm wavelength) employing an InGaN-based active layer, high emission efficiency is obtained at low current densities. A possible reason explaining such a result is that the contribution of localized energy levels originating in indium compositional fluctuations to device emission leads to high emission efficiency. In other words, the potential also varies spatially depending on the spatial variations (fluctuations) of the indium composition in the InGaN-based active layer. An increase in the current density, however, causes carriers to overflow from the localized energy levels, causing the regions surrounding the localized energy levels to contribute to emission. This is responsible for a decrease in the emission efficiency as a whole at high current densities.
According to results of studies on how to minimize decrease in emission efficiency, it is desirable to lessen fluctuations in the indium composition in the InGaN layer, in order to minimize decrease in emission efficiency at high current densities. Lowering the indium composition will reduce fluctuations in indium composition. But merely lowering the indium composition to reduce indium compositional fluctuations causes the emission wavelength to shift from the desired wavelength.
In order to have light-emitting devices emit light at desired wavelength the InGaN well layer thickness must be increased, in addition to lowering the indium composition. Nevertheless, as described in Applied Physics Letters, Vol. 78, 2001, p. 2617 and other cited literature, it is well known that in InGaN-based light-emitting diodes employing a (0001) sapphire substrate, an increase in the well-layer thickness causes LED output power to diminish.